When atomic parameters, for example, coordinates of atoms in the unit cell or populations of different sites in the lattice of the crystalline phase are altered, this change affects relative intensities and/or positions of the Bragg peaks that correspond to this phase. For example, the presence of a crystalline phase is manifested as a set of discrete intensity maxima - the Bragg reflections - each with a specific intensity and location. What makes the powder diffraction experiment so powerful is that different structural features of a material have different effects on various parameters of its powder diffraction pattern. Powder diffraction data are customarily recorded in literally the simplest possible fashion, where the scattered intensity is measured as a function of a single independent variable - the Bragg angle. Since many materials can only be prepared in a polycrystalline form, the powder diffraction experiment becomes the only realistic option for a reliable determination of the crystal structure of such materials. 1 The quality of the powder diffraction pattern is usually limited by the nature and the energy of the available radiation, by the resolution of the instrument, and by the physical and chemical conditions of the specimen. Although powder data usually lack the three-dimensionality of a diffraction image, the fundamental nature of the method is easily appreciated from the fact that each powder diffraction pattern represents a one-dimensional snapshot of the three-dimensional reciprocal lattice of a crystal. The powder diffraction experiment is the cornerstone of a truly basic materials characterization technique - diffraction analysis - and it has been used for many decades with exceptional success to provide accurate information about the structure of materials.
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